Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac rhythm management (CRM) devices such as implantable pacemakers and implantable cardioverter defibrillators (ICDs). The devices are used to treat patients using electrical therapy and to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. Implantable devices may also include electrical leads that are either separate from, or connected to, a CRM. Electrical leads connected to CRM devices are located in or near a heart to provide electrical therapy to the heart. The electrical leads are also in communication with sense amplifiers of the CRM devices to monitor electrical heart activity within a patient. Other examples of implantable medical devices include implantable insulin pumps or devices implanted to administer drugs to a patient.
Congestive heart failure is a disease that causes the ventricles of the heart to have a reduced ability to contract which results in an inadequate amount of blood being pumped into circulation. Because blood is being pumped away from the lungs at a reduced rate, fluid may build up in a patient's lungs and cause difficulty in breathing. As a patient's condition worsens, the patient may develop a tendency to rest in an elevated posture to reduce the fluid buildup in his or her lungs. Some CRM devices provide electrical therapy to treat congestive heart failure. The present inventors have recognized a need for improved monitoring of the condition of a congestive heart failure patient.